Method of making magnetic cores



0d 11, l949 J. G. FORD Erm.

METHOD 0F MAKING MAGNETICv CORES Filed Dec. 15, 1945 mmf d Nm, mm Nm., mm S, QW S WITNESSES: W/

` :i-Patented Oct. ll, 1949- burgli, Pa., asdgnors to Westinghouse Electric- Corporation, East Pittsburgh, Pa., a'corporation of Pennsylvania Application December 15, 1945, serial No. .635,312

4 claims. (ci. is4-'sm1 This invention relates to magnetic cores and in particular Ito a process for producing magnetc cores of predetermined structure.

. For many applications, it is desirable t0 produce from sheet magnetic material, magnetic vcores having a predetermined spacing between adjacent laminations. Such spacing is required in order to secure the most emcient structure and the maximum space factor with respect to the amount of magnetic material per unit volume of core. Numerous electrical applications require cores composed of sheets of magnetic material with a predetermined thickness `of resin or other insulating material or binder present inthe spaces between the sheets of magnetic material.

Great difficulties have been encountered in preparing cores with a uniform spacing between laminations. The difficulties increase greatly when the magnetic sheet material is extremely thin, particularly less than 5 mils in thickness.

According to this invention, a carefully controllable spacing between laminations forming a core is secured by applying to sheet magnetic material a coating of predetermined thickness composed of a readily depolymerizable and evaporable resin. By subjecting the core so prepared to heating at an elevated temperature. the resin is depolymerized and then caused to evaporate thereby leaving the predetermined spacing between l-aminations in the assembled core.

The object of this invention is to provide a process for producing a predetermined space between laminations in awound core.

A further object of the invention is to provide for building cores composed of a plurality of laminations with a predetermined spacing between laminations.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing in which:

Figure 1 is a schematic view illustrating the complete process of building magnetic wound cores, and

Fig. 2 is a greatly enlarged fragmentary crosssectional view through a lan/iination produced in accordance with the invention.

We have discovered that certain selected resinous materials may be applied to the surfaces of magnetic material to provide a uniform coating of predetermined thickness on the surfaces thereof. By forming magnetic cores from the resin coated magnetic material. the laminations will -2 be uniformly separated in accordance with the thickness of coating on the surfaces of the laminations. By employing for Vthe coating :a resin that is capable of readily depolymerizing at cer- Y tain elevated temperatures into monomers, di-

mers, or the like having a low vapor pressure, the coating can be eliminated by volatilization from between the laminations. thereby leaving a core having the predetermined open spacing between successive laminations.

A number of resinous materials are especially suited for the practice of this invention. Certain exacting requirements have been most favorably fulfllled by the use of lthe polymerized derivatives of acrylic acid, methacrylic acid and other alkyl derivatives of acrylic acid. The term polyesters of acrylic acid is employed herein to indicate this clam of resinous materials. It has also been found from experience that while a number of other resins depolymerize almost completely at elevated temperatures and the monomers or lower polymers so produced can be readily removed by evaporation, the polyesters of acrylic acid leave substantially no carbonaceous residue,

whereas other depolymerizable and evaporable resins frequently leave some carbonaceous residue. A carbonaceous residue is highly undesirable since magnetic materials are unfavorably affected in subsequent operations by even small amounts of carbon. For example; a preferentially oriented silicon iron containing 3.25% of silicon contains a carbon content of less than 0.01% and any increase of the carbon content above this value would result in a deterioration of the magnetic properties of the material. Even traces of carbon on the surfaces of the magnetic material during annealing will tend to increase the carbon content of the silicon iron to undesirable proportions.

Among the polyesters of acrylic acid that have been employed with success are polymethyl methacrylate, polyethyl methacrylate, polypropyl methacrylate, and polybutyl methacrylate. The higher polyesters depolymerize more rapidly than the lower ones; thus polybutyl methacrylate has been employed with less time required during heat-treatment to produce a-spaced core than by using lower polyesters. Methyl acrylate and tertiary polybutyl acrylate Aand other acrylates are suitable for the purpose of this invention.

It is feasible to use other depolymerizing resins for coating laminations if a. vacuum oran inert atmosphere or both are presentduring the depolymerizing heat treatment of the resin coated laminations to avoid the formation of a carbonfound tobe 92%, of metal with 8% way comprise cycloparailin polymers, cyclopentadiene polymers, coumarone-indiene resins and polyisobutyleneresins. y

When we refer to depolymerizingffV it is intended to cover the decomposition of high poly-v mers to both monomers and readily volatile low polymers such as dimers, trimers, and

low polymers.

The resin is preferably applied to the magnetic material in solution in -a volatile solvent. The solvents preferably, though not necessarily, should boil below 100 C. to facilitate prompt drying. Suitable solvents to be employed either alone or in mixtures, are ketones, such as acetone, isophorone, and methyl ethyl ketone, hydrocarbons,.such as toluene, chlorinated hydrocarbons such 'as trichlorethylene and fatty acid esters such as ethyl acetate. For` thin wire or strip, the

solutions are fairly dilute, for example with less than 10% solids, in order to control the thickness of the applied coating with greater accuracy. The following examples are typical of the practice of the invention:

Example I A solution of polymethyl methacrylate was prepared -by dissolving the resin in acetone to produce a 10% solution. Two-mil thick strips of` silicon iron ,were coated -by dipping into the solution and wound into cores. The cores were heat treated at a temperature of 350 C. for several hours. After annealing, the space factor was air space between laminations. Example II` Suilicient polybutyl methacrylate was dissolved Example III Polymethyl methacrylate was dissolved in g acetone to produce a solution having a 25% solid contact. When applied to two-mil silicon iron,

al thin even lm of the resin was deposited on g drying. The material was wound into c ores, and

after being heat treated at 500 core was found to have a space factor of 91%.

applied by dipping temperature of 450 forated from the C., the resulting Example IV Sufiicient polypropyl methacrylate was dissolved in benzol to produce a 10% solids solution. iFive-mil silicon iron was coated with the solution and-a. wound core was prepared after the solvent had evaporated. After heat treating at a C., all of the resin had evaplaminations and the resulting core had a space factor of 94%.

` By applying solutions of less solids content, `thinner coats of resin may be produced by a simple dipping operation. The solutions of depolymerizable resins may be applied by spraying `or by roll coating, by doctor blad or the like. In some cases the depolymerizable resin may be or brushing while in a molten state below the depolymerization temperatures that is,the core comprises 92% l in toluene to produce a 5% solution. One-mil thick silicon iron was coated by dipping in the solution. The solution dried in a minute and the magnetic strip was wound into cores and heat treated at a temperature of about 350 C. The 1 space ,factor in the cores after annealing was 289%. l n

benets. In cores having laminations spaced by a. film of resin, the comminuted refractory material will maintain a. suitable separation of the laminations after the resin has been evaporated out. For example when a stack of cores is being annealed, the laminations of cores at the bottom of the stack will not be pressed together, even at elevated temperatures, if refractory particles are present between laminations. A. further advantage is that adhesion of laminations is prevented. 'Ihe following examples are given by way of illustration, all parts being by weight:

Example V VParts Polymethylmethacrylate 10 Acetone Silica. powder, 325 mesh 10 The polymethylmethacrylate was dissolved in the acetone and the silica powder was stirred in to produce a suspension. An even thin lm of the suspension was= applied to two-mil thick sheet magnetic material, and after drying the sheet magnetic material was wound into a. core and annealed to remove the resin. The space factor of the core was 94%.

Example VI The mica powder was suspended in the polymethylmethacrylate solution. After applying to a magnetic strip of a thickness of 5 mils, and dried, the coated strip was wound into a core and heat-treated to remove the resin. The cores had a 95% space factor. The laminations were very uniformly spaced apart. f

The suspension, prepared` as in Examples V and VI, was applied to 3 mil strip. The strip was wound into cores and, after heat treatment to depolymerlze and evaporate the resin, the cores were found to have a 92% space factor. The magnesium carbonate decomposed during the reaction to give oil. carbon dioxide, leaving a residue of ne magnesium oxide particles to function as lamination spacing.

Many inorganic refractory solids in nely comminuted form may be added to the resin solutions. For instance, besides mica and silica, talc, asbestos, bentonite, pumice, glass powder, and aluminum oxides may be employed. Partially heat reactive solids may be added if a refractory residue is produced. Besides the magnesium carbonate of Example VII, zinc carbonatemay be so added.

The amount of the inorganic solid material may vary from about 5% to 250% of the weight of the solid resin lm. The iineness of the solid material is such that the particles have dimensions not greater than the desired spacing between laminations. For most purposes, 200 to 400 mesh powders will be satisfactory.

Various forms of magnetic material may be treated in accordance with this invention to provide a spacing coating of depolymerizable resin. While, for most applications, magnetic material is employed in sheet form, use is frequently made of magnetic material in the form of wire or other shapes, and the invention may be applied thereto regardless of the shape thereof.

Referring to Fig. 1 of the drawing, there is illustrated a typical process for producing cores in accordance with the present invention. The magnetic material I is unwound from a supply roll mounted on the stand l2 and passes into a tank I4 containing the solution of depolymerizable resin such as the polyester of arcylic acid, with or without solid refractory additions, prepared as described. The strip material passes over guide rolls I8 and thence below the surface of the solution I6 under the guide roll 20 and thence it passes out of the solution to squeeze rolls 22 and 2| for maintaining an even thickness of solution on the strip I0. The squeeze rolls 22 and 2l may be composed of a resilient material such as rubber or the like maintained under a given pressure which will remove any excess of the layer of solution on the surface of the strip. The solution I6, including a volatile solvent which will readily evaporate when exposed to the atmosphere, will provide an adherent resin coating upon the evaporation of the solvent. The coating of the polyester of acrylic acid is suflcient adherent to the magnetic strip to withstand subsequent processing in producing cores. While in many cases the coated strip l0 may dry sufficiently in a few minutes in the open atmosphere while passing from the tank Il to the core forming operation, it may be desirable in some cases to pass the coated strip through a drying oven or exhaust hood 26 for removing the solvent vapors to avoid any danger of fire or the like where the solvent forms a combustible vapor.

The magnetic sheet material with the adherent coating of the polyester of acrylic acid may be stamped or'punched into predetermined shape and the punchings or stampings assembled into a core. The formation of a wound core for which the space factor requirements are particularly critical is further illustrated in Fig. 1. A pair of tensioning and guide rolls 28 receive the resin coated strip l0 from whence it passes to a mandrel 3l) having a radius of approximately oneeighth inch at the corners, where the strip is wound into a rectangular core 32. A pressing roll 3l is employed to compact the laminations forming the core 32, so that there may be secured the highest space factor possible.

After winding, the core 32 is subjected to a strain annealing operation in the furnace 36 in order to remove the strains set up in the magnetic material by the core forming operations such as the winding operation. The temperatures required for strain annealing are rarely less than 600 C. and may be as high as' 1200 C. and higher. It is desirable to remove the resin coating during the strain annealing operation. By employing a suitable atmosphere such as hydrogen or nitrogen gas within the strain anneal furnace 36, and by controlling the application of heat to the cores being annealed, the resin applied to the laminations may depolymerized and evaporated at a temperature of from about 350 C. to 500 C. The polyesters of acrylic acid and other resins may be evaporated after depolymerization, without leaving a trace of carbon, by suitably controlling the rate of temperature rise and the oven atmosphere. If the strain annealing oven is permitted to heat up too rapidly and excessive temperatures are reached while the resin is depolymerizing, pyrolysis of the resin may occur resulting in the formation of substantial amounts of carbonaceous residue which would be detrimental to the magnetic material. Ordinarily, an hour within the Y temperature range of 350 C. to 500 C. is sumcient to remove the entire coating of resin on.

the surfaces of the laminations. An abundant circulation of gas to remove the volatile monomers and low polymers should be provided. The gases may be run through a condenser to condense the resinous materials, and the gases may then be recirculated. Obviously the strain annealing furnace may be operated at a reduced pressure such as 100 millimeters of mercury if polystyrene or other depolymerizing resin subject to the formation of a carbonaceous residue is employed.

After strain annealing, the laminations of the annealed cores 32 will be found to be separated by spaces of great uniformity of thickness. The laminations will not be subject to sticking or adhering as has occurred when spacing means are not employed or when improper spacing means are made use of. Furthermore the space factor of all the cores will be found to be quite uniform. With the application of refractory solids in the depolymerizable resins, the spacing will be maintained remarkably uniformly. No adhesions of laminations can occur in this latter case.

'Ihe strain annealed cores are then impregnated in a vessel 38 with a suitable bonding resin to unite the laminations into a homogeneous bonded core unit. Numerous resins suitable for this purpose have been disclosed. Thus, mixtures of polyvinyl acetate and polyvinyl acetal with phenol-formaldehyde as disclosed in Ford Patent 2,372,074 may be employed. Due to the uniform space between laminations, the impregnating resin will readily penetrate and fill the entire space, thereby enabling the attainment of an excellent bond. In case powdered solids have been employed, it will be found that the resin will penetrate the interlaminar spaces easily because of the uniformity of the spaces. The resins impregnated cores 32 are then heat treated in an oven ll to evaporate the solvent, if any be present, and to polymerize the bonding resin into a fully hardened condition. The resin bonded core may then be subjected to machining by setting it up in a jig l2 whereit may be cut by a saw 44. The cores may be cut or machined by means of milling cutters or resin bonded grinding wheels. Thereafter the cut core may be placed in the jig I6 and the cut faces machined with a grinding wheel Il to provide for a plane and flat face free from rough burrs. If desired, the core may be etched in an acid to remove ne burrs or other undesirable short-circuiting metallic particles between laminations that may be produced by the machining operations.

Referring to Fig. 2 of the drawing, there is illustratcd a greatly enlarged view of the sheet of magnetic iron Il. The magnetic iron is provided with I5 an integral film 5l of a. refractory insulating mabe successfully n tion of signicant amounts of at a given temperature of 350 resin coating carrying the refactory solids, forming the coated sheet material into a core and heati ing the core at the given temperature to depoly. Imerize and evaporate the resin, leaving the ilnely divided refractory solids to teria] which is unaffected by the various machining and treating operations shown on Fig. 1. The refactory film 50 is ordinarily produced during the manufacture of the sheets of silicon iron. It may be composed of the reaction product of magnesium oxide and silica with the silicon iron or it may be composed of phosphoric acid reaction products. After coating with the depolymerizable resin solution in the tank i4 and drying in the oven 26, a coating 52 of predetermined thickness of the depolymerizable resin will be present on the surfacesof the sheet I which may then be wound, bent, or otherwise formed into a core. The successive laminations I0 will be separated from one another by two layers 52 of the depolymerizable resin. After heat treatment, the successive laminations will have an intervening open space present equal to twice the thickness of the coating 52.

Since certain changes in carrying out the above processes and certain modifications in the compositions which embody the invention may be made without departing from its scope, it isy in- -tended that all the matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In the process of producing a. magnetic core with a predetermined space factor, the steps comprising applying to a surface of magnetic sheet material a coating of a predetermined thickness 3 of a polyester of acrylic acid selected from the group consisting of the methyl, ethyl, propyl and.- l 1 butyl polyesters, i and evaporated without the formation of significant amounts of carbon l temperature of 350 coated sheet material into a core and heating the core at the given temperature to depolymerize and evaporate the polyester, 1 with the predetermined space factor.

capable of being depolymerized C. and higher, forming the thereby producing a core 2. In the process of producing a magnetic core l with a predtermined spacefactor, the steps comg prising applying to a surface of magnetic sheet material a thin layer of a solution composed of a readily evaporable solvent and a resin, the solution carrying finely divided refractory solids of a diameter not in excess of the desired space `between laminations, evaporating the solvent to leave on the sheet material a coating of a prel determined thickness ofa resin capable of being depolymerized and evaporated without the formacarbon by heating C. and higher, the

' proportion of a resin therein, the solution thereby producing a core with the predetermined space factor. I

3. In the process of building a wound core with a predetermined Space factor, the steps comprising, applying to sheet magnetic material a layer of a selected thickness of a solution composed of a readily evaporable solvent and a selected minor having suspended therein refractory solids of a neness of from 200 to 400 mesh, the resin having the property of depolymerizing ata. given temperature of 350 C. and higher substantially completely and evaporating without the formation of signincant amounts of carbon, evaporating the solvent from the applied layer to provide a coating oi' resin of predetermined thickness on the sheet material, the resin coating carrying the refractory solids, winding the coated sheet material into av core, and heat treating the wound core at the temperature of 350 C. and higher for a suiiicient length of time to depolymerize and evaporate the Y resin and to leave the refractory solids for spacing by heating at a given space the laminations, so 2,116,318

lthe laminations, thereby providing a wound core having the predetermined space factor.

4. In the process of building a wound core with a predetermined space factor, the steps comprising, applying to sheet magnetic materiala layer of a selected thickness of a solution composed of a readily evaporable solvent and a selected minor proportion of a polyester of acrylic acid selected from the group consisting of methyl, ethyl, propyl and butyl polyesters, the polyester having the property of depolymerizing at a given temperavture of 3 50* C. and higher substantially completely and evaporating without the formation of sig- Y nificant amounts of carbon, and from 5% to 250%, 'based g' on the K, weight of the polyester, of from 200 to 400 mesh refractory solids, evaporating the solvent from the applied layer to provide a coating of polyester of predetermined thickness on the sheet material carrying the refractory solids therein, winding the coated sheet material into a core.v and heat treating the wound core at the given temperature for-a suiiicient length of time to depolymerize and evaporate .the polyester and to leave the refractory solids for spacing the laminations, thereby providing a wound corefhaving the predetermined space factor.

l JAMES G. FORD.

FRITZ J. NAGEL.

REFERENCES CITED UNITED STATES PATENTS Number Y -Name Date 1,682,364 Ballantine Aug. 28, 1928 1,842,970 Hovey Jan. 26,1932

Miles, Jr May 3, 1938 

